1. Field of the Invention
This invention relates to improvements of image display quality and productivity of a spatial light modulator to be used on flat panel displays, optical operation elements and video projectors.
2. Description of the Related Arts
As well known, it is possible to perform an incoherent-coherent light conversion or a reverse operation thereof by using a spatial light modulator. Thus, as applications of the spatial light modulator, there are considered a parallel data processing and a direct image processing. Further, the spatial light modulator is applicable to a display system of the video projector by amplifying an intensity of the light beam.
FIG. 1 is a sectional side view of a spatial light modulator of the present invention.
At first, a description is given of an outline of the spatial light modulator 1 in reference to FIG. 1.
Generally, a spatial light modulator 1 has a writing side having a photoconductive layer 13 where a writing light beam of an image is inputted, and a reading side having a photomodulation layer 10 made of a liquid crystal layer as a modulation material where a reading light beam is inputted to read out the image recorded on the photoconductive layer 13. The reading side and the writing side are optically shielded to each other by a light shielding layer 12 so as to prevent the reading light beam from invading the writing side.
Specifically, the spatial light modulator 1 comprises a glass substrate 15, a transparent electrode 14, the photoconductive layer 13, the light shielding layer 12, a reflecting layer or a dielectric mirror 11, the photomodulation layer 10, a transparent electrode 16 and a glass substrate 17 stacked in this order, and a driving power source 18 is connected between the transparent electrodes 14, 16.
Next, a description is given to the operation of the spatial light modulator 1.
The writing light beam carrying desired information is inputted into the photoconductive layer 13 in a direction of an arrow F1 through the glass substrate 15 and the transparent electrode 14. In the photoconductive layer 13, the impedance of the photoconductive layer 13 is changed in accordance with the intensity distribution of the writing light beam, so that the photoconductive layer 13 has a conductive distribution corresponding to the intensity distribution of the writing light beam as it scans the photoconductive layer 13. Thus, the voltage from the driving power source 18 is applied to the photoconductive layer 13 correspondingly with the conductive distribution, i.e., a two dimensional intensity distribution of the writing light beam. At that time, the arrangement of the liquid crystal molecules in the photomodulation layer 10 is also changed correspondingly with an electric field distribution applied to the photomodulation layer 10 caused by the conductive distribution of the photoconductive layer 13.
On the other hand, the reading light beam is inputted into the photomodulation layer 10 in a direction of an arrow F2 through the glass substrate 17 and the transparent electrode 16. Thus, the reading light beam is modulated correspondingly with the electric field distribution in the photomodulation layer 10. The reading light beam modulated is reflected by the dielectric mirror 11, and is outputted in a direction of an arrow F3. Incidentally, when a crystal or a liquid crystal supported by a support (for instance, a liquid crystal film) is employed as the photomodulation layer 10, the glass substrates 15, 17 or one of them may be omitted.
As mentioned in the foregoing, the light shielding layer 12 is used for preventing the reading light beam from invading the photoconductive layer 13 through the dielectric mirror 11 and from disturbing the image charge recorded in the photoconductive layer 13, otherwise the contrast ratio and resolution of the image read out will be degraded.
When the spatial light modulator 1 is employed in the video projector, the leakage light beam from the reading light beam will cause an adverse effect in the image quality, in particular, when the photoconductive layer 13 which has a high sensitivity to the wavelength of the leakage light beam, is employed in the spatial light modulator 1, the leakage light beam causes not only degradations of the contrast ratio and resolution of the image displayed but also image dropout thereof.
Generally, in order to avoid the problems mentioned above, the light shielding layer 12 is employed, but, it is preferable to form the light shielding layer 12 by using a material having a high electrical resistivity to prevent the degradation of the resolution of the image caused by the light shielding layer 12 itself.
However, such a material having both a high electrical resistivity and a high light blocking function, is not easily available.
As seen in Japanese Patent Laid-Open Publication of H2-501334/90, a material of CdTe is used as the light shielding layer, but the material of CdTe has the problems of high cost and toxicity. Further, the material has a drawback of poor adhesion, thus it requires intermediate layers to enhance adhesion between the shielding layer and the photoconductive layer and between the shielding layer and the reflecting layer. This causes a complex structure of the spatial light modulator, so that a possibility of generating defects of the spatial light modulator increases in the production process.